This invention relates to a control device for an actuator, and more particularly to a control device for an actuator which is adapted to keep a discharge pressure of a variable discharge pump increased in an amount corresponding to a pressure set by a regulator as compared with a load pressure.
FIG. 1 is a circuit diagram showing a typical power shovel which has been conventionally known in the art. The conventional power shovel includes a variable discharge pump 1 which is associated with or operatively connected to a power source (not shown) such as an engine or the like and connected on a discharge side thereof to a high pressure flow passage 2. The high pressure flow passage 2 is then connected to an input port 5 of a first change-over valve 4 connected to a boom cylinder 3, an input port 8 of a second change-over valve 7 connected to a bucket cylinder 6, and an input port 11 of a third change-over valve 10 connected to a spin motor 9 in turn.
When the first change-over valve 4, second change-over valve 7 and third change-over valve 10 each are at a neutral position shown in FIG. 1, the corresponding input ports 5, 8 and 11 are kept closed. Then, when each of the change-over valves 4, 7 and 10 is changed over to any one of both lateral positions, variable orifices 12 to 14 are rendered open. A degree of opening of each of the variable orifices 12 to 14 is controlled depending on the amount of changing-over of the change-over valve corresponding thereto. The variable orifices 12, 13 and 14 are connected on a downstream side thereof to pressure compensating valves 15, 16 and 17, respectively.
The pressure compensating valves 15 to 17 are arranged so as to communicate on a downstream side thereof with a feed port 18 of the first change-over valve 4, a feed port 19 of the second change-over valve 7 and a feed port 20 of the third change-over valves 10, respectively. The feed ports 18 to 20 of the change-over valves are kept closed when the change-over valves 4, 7 and 10 are at the neutral position. Then, when the change-over valves 4, 7 and 10 are changed over to any one of the lateral positions, the feed ports 18 to 20 are adapted to correspondingly communicate with any one of actuator ports 21 and 22, any one of actuator ports 23 and 24, and any one of actuator ports 25 and 26 in correspondence to changing-over of the valves, respectively. At this time, the remaining ones of the actuator ports 21 and 22, 23 and 24, and 25 and 26 are adapted to communicate with tank passages 27, 28 and 29, respectively.
The first, second and third change-over valves 4, 7 and 10 are formed with load detection ports 30, 31 and 32, respectively. The load detection ports 30 to 32 are kept communicating with the tank passages 27 to 29, respectively, when the first to third change-over valves 4, 7 and 10 are at the neutral position. Then, when the first, second and third change-over valves 4, 7 and 10 are changed over to any one of both lateral positions, the load detection ports 30 to 32 each are permitted to communicate with an actuator port on a high pressure side.
The pressure compensating valves 15, 16 and 17 are provided on both sides thereof with pilot chambers 15a and 15b, pilot chambers 16a and 16b, and pilot chambers 17a and 17b, respectively. The pressure compensating valves 15 to 17 act to guide a pressure on an upstream side thereof to the one pilot chambers 15a to 17a, respectively, as well as a load pressure on the load detection ports 30 to 32 to the other pilot chambers 15b to 17b, respectively. The load pressure thus guided to or introduced into the other pilot chambers 15b to 17b is selected by means of a plurality of shuttle valves 33, resulting in a maximum load pressure in each of the circuit systems being guided to or introduced into each of the other pilot chambers 15b to 17b. The other pilot chambers 15b, 16b and 17b are provided thereon with springs 34, 35 and 36, respectively, which are adapted to generate elastic force acting on the pilot chambers 15b to 17b.
Thus, the pressure compensating valves 15 to 17 carry out a control operation in such a manner that the pressure on the upstream side of the valves 15 to 17 is kept at a level increased by an amount corresponding to the elastic force of the springs 34 to 36 as compared with the maximum load pressure in the circuit systems.
The maximum load pressure selected by the shuttle valves 33 is introduced into a pilot chamber 37a which is one of two pilot chambers 37a and 37b of a valve 37 for controlling the variable discharge pump 1. The pilot chamber 37a is so constructed that elastic force of a spring 38 acts thereon. To tile other pilot chamber 37b of the valve 37 is guided a pressure in the high pressure flow passage 2 or a discharge pressure of the variable discharge pump 1. Such construction results in the valve 37 being changed over between a normal position (a) and a changed-over position (b) depending on a relative difference between the discharge pressure of tile variable discharge pump 1, and the maximum load pressure and the elastic force of the spring 38.
When the valve 37 is changed over to the normal position (a), a control cylinder 39 for controlling a tilting angle of the variable discharge pump 1 is permitted to communicate with a tank T to keep a flow rate of fluid discharged from tile pump 1 maximum; whereas, at tile changed-over position (b), a pressure of the pump is introduced into tile control cylinder 39 to decrease the flow rate of fluid from the pump 1. The valve 37 is adapted to determine a degree of opening thereof while moving between the normal position (a) and the changed-over position (b).
Reference numeral 40 designates a main relief valve, which serves to set a maximum pressure in each of circuit systems of the boom cylinder 3, bucket cylinder 6 and spin motor 9.
As will be noted from the foregoing, the conventional control device thus constructed is the load-sensing type. Thus, the variable discharge pump 1 discharges a pressure increased by an amount corresponding to the elastic force of the spring 38 as compared with the maximum load pressure. The pressure compensating valves 15 to 17 of the circuit systems control a pressure on the downstream side of the variable orifices 12, 13 and 14 of the first, second and third change-over valves 4, 7 and 10 depending on the maximum load pressure. This causes a pressure difference between a frontward side of each of the variable orifices 12 to 14 and its rearward side to be constant, to thereby feed fluid ill an amount proportional to the amount of changing-over of each of the change-over valves 4, 7 and 10 to each of actuators.
The variable discharge pump 1, as described above, is associated with or operatively connected to the engine (not shown), so that the number of rotations of the former is determined depending on the number of rotations of the latter.
In the conventional control device constructed as described above, the elastic force of the spring 38 provided on the valve 37 is rendered constant, so that gain characteristics in flow control by the control device are not varied as shown in FIG. 2.
As will be apparent from FIG. 2, the conventional control device permits a flow control range thereof to be sufficiently increased as indicated at .alpha. when the number of rotations of the engine is kept at an increased level. However, when the number of rotations of the engine is reduced to cause a maximum flow rate of fluid discharged from the variable discharge pump 1 to be Q.sub.1, the control device causes the control range to be decreased as indicated at .beta., because the gain characteristics are not varied as described above.
Such a decrease in flow control range to .beta. which is encountered when a flow rate of fluid discharged from the variable discharge pump 1 is reduced causes a disadvantage of deteriorating an operational feeling of the change-over valves as compared with an increase in flow control range to .alpha. which ensures an increase in flow rate of fluid discharged from the pump 1.